1. Field of the Invention
The invention relates to a method and apparatus for the control of an image display screen of the "memory effect" type. It is aimed in particular at reducing a so-called "pulsed discharge current" in order to reduce or even eliminate its harmful effects. The invention relates especially (but not exclusively) to screens whose picture elements are cells having two stable states, namely the "lit" state and the "extinguished" state.
The term "memory effect" is understood to mean the effect that enables cells to preserve the "lit" or "extinguished" state when the signal that has produced this state has already disappeared.
2. Discussion of the Background
Taking for example the case of an alternating plasma panel (PP) with two crossed electrodes to define a cell, this type of screen has cells with two stable states and benefits from a "memory effect" as described especially in the patent FR 2 417 848. A PP of this kind is described here below with reference to FIG. 1.
The PP has an array of electrodes Y1 to Y4 called "row electrodes" intersected with a second array of electrodes called "column electrodes" X1 to X4. To each intersection of row and column electrodes there corresponds a cell C1 to C16. These cells are thus arranged in rows L1 to L4 and in columns CL1 to CL4.
Each row electrode Y1 to Y4 is connected to an output stage SY1 to SY4 of a row control device 1 and each column electrode X1 to X4 is connected to an output stage SX1 to SX4 of a column control device 2.
The working of these two control devices 1, 2 is controlled by an image management circuit 3.
For each cell, the voltage applied at a given time to a given cell C1 to C16 is the one resulting from the difference in potential, at this given instant, between the signals applied to the row electrode Y1 to Y4 and the column electrode X1 to X4 which define this cell.
Each output of the row control device 1 delivers voltage square-wave signals called "sustaining signals" SS on which addressing signals may be superimposed.
In a PP, each cell comprises a space occupied by a gas. By applying sufficient voltage between the two electrodes that define a given cell, there is prompted an electrical discharge in the gas and an emission of light by this cell.
In an alternating PP, the electrodes Y1 to Y4 and X1 to X4 are covered with a dielectric material and are therefore not in direct contact with the gas or with the discharge. Consequently, whenever there is a discharge in the gas, electrical charges collect in the dielectric at the two electrodes that define a cell in which a discharge occurs. These charges persist after the end of the discharge and enable the constitution of a "memory effect" for their presence at the level of a cell C1 to C16 enables the prompting of a discharge in this cell with the application of a voltage lower than that which will be necessary when these charges are absent.
The cells C1 to C16 which have such charges are said to be in the "recorded" or "lit" state. The other cells which require a higher voltage to produce a discharge are said to be in the "erased" or "extinguished" state. This effect is used by means of the sustaining signals SS to activate the cells C1 to C16 which are in the "recorded" state, namely to prompt discharges in these cells without modifying their state or modifying the state of the cells which are in the "erased" state.
The cells C1 to C16 are put into the "recorded" state or the "erased" state by means of addressing operations that are often performed row by row, namely for all the cells belonging to one and the same row L1 to L4 (in other words, for all the cells C1 to C16 defined by one and the same row electrode Y1 to Y4) and then for all the cells of another row.
FIG. 2 gives a simplified view, in the rows a, b, c, d, of the sustaining signals applied simultaneously to all the row electrodes Y1 to Y4 of a PP. It illustrates the addressing operations performed on these row electrodes: the rows a, b, c, d represent respectively the signals applied to the row electrodes Y1, Y2, Y3, Y4.
The sustaining signals are constituted by a succession of voltage square-wave signals set up on either side of a reference potential Vo which is often the ground potential. These square-waves vary between a negative potential V1 where they have a plateau or steady level and a positive potential V2 where they have another steady level. These positive and negative potentials V2, V1 with respect to Vo may each have for example a value of 150 volts. The reference potential Vo is applied to the column electrodes X1 to X4 in such a way that the application of the sustaining signals develops alternately positive and negative voltages of 150 volts, in the example, at the terminals of the cells. These voltages generate discharges in all the cells in the "recorded" state at each reversal of polarity, namely at each positive or negative transition tp, tn of the sustaining signals.
The sustaining signals have a period P which is currently in the range of 20 microseconds. This is a period during which the addressing of all the cells defined by a selected row electrode is done.
The addressing operations are managed by the image management device 3. They consist, for example, of the superimposition of the specific addressing signals on the square-waves that form the sustaining signals. Each row output stage SY1 to SY4 comprises, for example, to this effect a mixing circuit (not shown) by means of which it receives the sustaining signals and the addressing signals that come from different channels.
Assuming that the addressing operation performed on the row electrode Y1 starts at an instant to, the signal applied at this instant solely to this row electrode is an erasure pulse tne (shown in dashes) with a voltage lower than that of a square-wave, which prompts the placing of all the cells connected to this row electrode Y1 in the "erased" state. Then, at an instant t1 where the signal has its positive steady level, a so-called recording square-wave CI (shown in dashes) is superimposed (positively) on this stage. This recording square-wave has the effect of placing all the cells connected to this row electrode in the "recorded" state, except for those whose column electrodes X1 to X4 deliver a so-called "masking" signal (not shown) that has the effect of inhibiting the effects of the recording square-wave CI.
This operation may be repeated for each of the following periods, at the instants t2 and t3, t4 and t5, t6 and t7 where the addressing operations are thus performed on the row electrodes Y2, Y3 and Y4. During an image cycle period or frame period, these operations are performed at least once. In fact they are performed several times to obtain half-shades in the image. In view of the large number possible of row electrodes such as the electrodes Y1 to Y4, a number which may be far greater than one thousand, the time needed to perform the addressing may lead to the addressing of several rows during one and the same period P.
During the period when a row is addressed, the sustaining signals applied to the other row electrodes generate discharges in the cells in the "recorded" state. These discharges are in phase with the transitions tp, tn. These discharges constitute currents Id set up in the cells which are in phase with the transitions tp, tn as shown in the row e of FIG. 2.
The sustaining signals are applied synchronously to all the row electrodes Y1 to Y4. The result thereof is a simultaneity of the discharges which may lead to substantial drawing of current which could have a deleterious effect on the quality of the image.
Indeed, PP may have more than a thousand row electrodes and more than a thousand column electrodes, which define more than a million parallel-supplied cells. Thus, the total discharge current produced by all the cells in the "recorded" state may attain considerable values that are difficult to provide by the electronic means, all the more so as this current must be built up in a very short time so as not to hinder the physical phenomenon of the discharge in the gas of the cells.
The total discharge current, called the pulsed current, may vary very greatly from one instant to another as a function of the contents of the image. Consequently, the quantities of current drawn, to which there has to be a response from the voltage sources or amplifiers or generators used to prepare the signals and voltages applied to the row and column electrodes, vary greatly in themselves.
Given the non-zero source impedances of the voltage sources and amplifiers as well as the access impedances to the cells (related in particular to the inductors and resistors of the connections), the voltage actually applied to a given cell depends on the total content of the image as does the quantity of light produced by this cell.
This may result in a major deterioration in the quality of the image.
It may even result in a deterioration of certain elements such as, for example, power transistors used at output of the column output stages X1 to X4 which, owing to this fact, have to be greatly oversized despite the technical drawbacks (increase in the space requirement, capacitance, etc.).
The problems raised by the high value of the pulsed discharge current tend to acquire all the greater importance as, at the present time, there is a development of matrix screens and especially alternating color PPs towards large sizes.
In order to overcome the above-mentioned drawbacks, it has been proposed in the prior art to reduce the frequency of the sustaining signals. This leads to a reduction of the average discharge current but not to a reduction of the pulsed discharge current because, for all the rows, the discharges occur simultaneously. Furthermore, this approach leads to a reduction of the addressing speed.
One known approach consists of the multiplication or oversizing of all or part of the elements used to supply the cells with voltage. This approach has the drawback, inter alia, of being costly.